This is already implemented - kind of. Plugin generates title from the selected fields (you can show it on the loop, single item, ...), you can display this plugin generated title with shortcode, but there is one catch. User still have to enter his own ad title even if you will not use it anywhere.
Good solution would be, that item title would be completely replaced by plugin generated title which would be generated after user clicks on publish button. Also it would be nice to have option to use this function only on selected categories - you may have a big general website where you would like to standardize titles from vehicle category, while you would like to leave option to enter custom titles to other categories like baby clothes.